Vibration damping means for sewing machines



April 10, 1956 J. G. ATTWOOD EIAL 2,741,198

VIBRATION DAMPING MEANS FOR SEWING MACHINES 4 Sheets-Sheet 1 Filed Feb. 26, 1953 .1. G. ATTWOOD ETAL 2,741,198

VIBRATION DAMPING MEANS FOR SEWING MACHINES Filed Feb. 26, 1955 4 Sheets-s 2 FIG.2

I H g J i I April 0, 1956 J. G. ATTWOOD ET AL 2,741,193

VIBRATION DAMPING MEANS FOR SEWING MACHINES Filed Feb. 26, 1953 4 Sheets-Sheet 3 April 10, 1956 J. G. ATTWOOD ETAL 2,741,198

VIBRATION DAMPING MEANS FOR SEWING MACHINES Filed Feb. 26, 1953 4 Sheets-Sheet 4 FIG- 5 43 8 United States Patent VIBRATION DAMPING MEANS FOR SEWING MACHINES John G. Attwood, Oak Park, and Clarence C. Smith,

Chicago, Ill., assignors to Union Special Machine 6on1- pany, Chicago, 11]., a corporation of Illinois Application February 26, 1953, Serial No. 339,111

7 Claims. (Cl. 112-4220) The present invention relates to sewing machines and more particularly to the damping or absorbing of torsional vibrations in high speed sewing machines.

It has been found that in the operation of sewing machines at high speeds there is a tendency to set up torsiona1 vibrations in the rotating shafts which serve to drive other machine elements back and forth at high speeds. When the shafts are rotated at certain speeds, generally termed critical speeds, they become dynamically unstable because of resonant effects as a result of which relatively large amplitude torsional vibrations are likely to develop. The creation of such vibrations is objectionable not only because of the noise which results but also because of the excessive wear of the parts and the difliculties encountered in the handling of the work and in the proper formation of stitches.

There are two principal types of forces which give rise to torsional vibrations in the rotating shafts of the machine. One of these is the inertial type which is due to the mass and acceleration of parts having reciprocatory or oscillatory or intermittent movement. The other is the impact type which arises when moving members, because of clearance tolerances, strike their confining members. Impact forces also arise from the striking of certain parts, such as the feed dog and presser foot, in the performance of their function. In a high speed sewing machine, torsional vibrations may arise at a number of points. For example, in moving a needle bar down and up, the rotating driving shaft starts the needle bar from a standstill at the top of its stroke, the starting force twisting the driving shaft in one direction. At the bottom of the stroke, the needle bar is slowed to a stop, thus twisting the shaft in the other direction. Two additional twists are imparted to the driving shaft in moving the needle bar from the bottom to the top of its stroke, so that the driving shaft receives two twisting cycles for each revolution thereof. These inertial twisting forces imparted to the shaft give rise to torsional vibrations. Similar torsional vibrations may be set up in the shaft by impact forces which arise when the needle bar and the associated moving parts strike their confining members in taking up the tolerances between the parts.

If the twisting or vibratory frequency coincides with the natural resonant or critical frequency of any independently vibratable part of the driving linkage or of the driving linkage as a whole, the amplitudes of the resulting torsional vibrations will be amplified, tending to cause extreme deflections and resulting in objectionable noise, wear, loosening of screws and even breakage of parts, and in poor sewing. The torsional vibrations may be considered as oscillating components of motion superimposed on the normal rotary motion of the shaft or other machine element. Normally a shaft and its associated driving connections are sufi'iciently stiff that the resonant or critical frequency thereof corresponds to a speed substantially higher than the normal operating speed of the machine. However, when a belt is used as a part of the driving connections, the flexibility thereof is sufiiciently 2,741,198 Patented Apr. 10, 1956 great that the resonant or critical frequency of the driving connections generally falls within or adjacent the operating speed range of the machine. Such coincidence or nearnes-s of the operating speed range and the critical or resonant frequency sometimes occurs in sewing machines employing gears or other linkage means between the shafts. Furthermore, since the motion of a driven part, such as a needle bar, is made up of a fundamental frequency and a number of harmonic frequencies, there is a possibility that one or more of these harmonic frequencies may coincide with the resonant frequency of the driving connections or a harmonic thereof.

In many sewing machines, the torsional vibrations set up in the top rotary shaft by reciprocation of the needle bar are much greater than similar vibrations set up in other parts of the machine. However, such vibrations are present elsewhere in a sewing machine, and while they may not produce undue noise, they do nevertheless tend to produce wear or loosening of screws and other parts and to intei'fere with proper sewing, so that it is frequently desirable to suppress these vibrations as well'as the vibrations set up in the top rotary shaft. For example, undesirable torsional vibrations are frequently set up in rotating element-s included in a sewing machine looper actuating mechanism.

In accordance with the foregoing, the principal object of the present invention has been to provide, in high speed sewing machines, novel and improved means for suppressing torsional vibrations.

More particularly, it has been an object of the invention to provide a high speed sewing machine with one or more dynamic torsional vibration dampers or absorbers for suppressing torsional vibrations in the machine.

A feature of the invention has been the provision of a novel and improved tunable dynamic torsional vibration damper for suppressing torsional vibrations in high speed sewing machines.

Other and further objects, features and advantages of the invention will appear from the following description.

A sewing machine constructed in accordance with the invention comprises a plurality of cooperating stitch forming and work feeding elements, driving'connections for these elements including a rotating shaft, means in the driving connections serving to apply periodic reversals of torsional force to the shaft, thereby tending to create torsional vibrations therein, and means for suppressing these torsional vibrations comprising an inertia member arranged to rotate with the shaft and resiliently coupled thereto. In accordance with a feature of the invention, means are associated with the inertia member for varying itsimoment of inertia by varying the location of a portion of the mass in relation to the center of rotation of the inertia member.

The invention will now be described in greater detail with reference to the appended drawings in which:

Fig. 1 is a longitudinal sectional view through a sewing machine constructed in accordance with the invention';

Fig. 2 is an end elevational view of the machine as seen from the left of Fig. l, a needle head cover being removed;

Fig. 3 is a top plan view of a portion of the sewing machine of Fig. 1 with a top cover member removed;v

Fig. 4 is an enlarged view taken along the line 4-4 of Fig. 1 and'shows a dynamic torsional vibration damper constructed in accordance with the invention and associated with the upper shaft belt sprocket;

Fig. 5 is a sectional view taken along the line 5-5 of Fig. 4; and Fig. 6 is an enlarged sectional view of the right end portion of the lower shaft with its combined hand and belt -=wheel and associated dynamic torsional vibration damper of Fig. 1.

Referring now to the drawings, the invention has been illustrated as embodied in ajfiat bed sewing machine of known type. -However, it should be understoodlthat the invention may be "applied advantageously to many other forms of high speed sewing -maohines.

. As show-n' in Fig. l, the sewing machine comprises a hollow base having a bottom wall and a topwall or cover 21 on which is supported a cloth plate 22. The hollow base contains an oil reservoir 23 anal carries certain elements of the stitch forming and work feeding mechanisms which are outside of the enclosed reservoir.

The frame of the sewing machine includes a hollow standard 24which communicates atitslower end with the hollow base, outside of -t-he reservoir 23, and which has integrallyiormed therewith a hollow overhanging arm 25-which terminates in a needle head 26. 7

An upper or needle bar shaft 27 of the machine extends'through the overhanging arm '25 and is journaled in bearing members 28 and 29 located,'respectively, at opposite ends'of the overhanging arm 25. A belt sprocket 30 is mounted on the end of the shaft 27 within the hollow standard 24, this sprocket being rigidly affixed to the shaft 27 by. means of a set screw 31 provided in a hub portion 32 of the sprocket. A needle bar 33 is mounted for -.vertical reciprocation in bushingsj34 and 35 provided in the needle head 26. At its lower end'the needle bar 33 is provided with a needle 36. T0 reciprocate the needle bar a crank member 37 is mounted-on the end of theshaft 27 within the needle head :26. A crank pin 38 carried by=the crank 37 provides a pivotal connection with the upper end of a pitman 39 which, at its other end, is pivotally connected to theneedle bar '33 by means of a=pin 40 extending from a split collar "41 clamped to the 'needle bar. The arrangement isgsuch that one complete reciprocation is imparted 'to'the needle bar 33 with each revolution of the shaft 27. p

The belt sprocket 30 is connected to a belt'sprocket 42 by means ofa toothed or ribbed belt 43. Sprocket 42 is mounted on a lower shaft 44 and is affi-Xed thereto by means of aset screw 45 provided in a hub portion '46 of the sprocket. The shaft 44 is supported within the base or the machine by suitable bearings including the bearings 47 and 48. One end of the shaft '44 extends beyond end wall49 of the base and has a'fiixed thereon, by means i of a set screw '50, acombined hand andhelt wheel "51 by which the machine 'may be operated either manually "or by'power. An oil slinging disc 52 is mounted on the shaft '44 so that the periphery thereof dips into the oil within the reservoir 23. *Through rapid rotation of the disc 52 during operation of the sewing machine, oil is dispersed in the form of a mist within the-reservoir portion 23 of the hollow base and is also thrown upward through a hollow tube 53 within the standard 24, thereby supplying lubricating oil to hearing member 29 and, through a tube 54, to the bearing 28 andthe moving elements within the head '26.

Atlooper ,is disposed within :an aocessible portion of the hollow base, outside .of the enclosed reservoir, and ;is arranged to cooperate .vthe needle 13.6 :in the formation of stitches in well known manner. The loop taking and needle avoiding movements ,of the I looper 60 areprovided through a mechanismactuated through "lower shaft 44. This mechanism includes an eccentric (not shown .on the shaft 44 and a pitman 61 ,having astrap at its upper end surrounding the eccentric and connected at its lower end, through a .ball and socket connection 7 62 to a horizontal arm 63 carried bya rock shaft 64, within the reservoir 23,. This shaft passes through the front wall ,of the base and has secured to it'scuter tend an upright arm 65 which, through an extensible Pitman rod 66, is connected by a strap 66' to a hallstud mounted on a-carrier 67 for the looper ;60. The carrier ,67 is arranged to swing on a stud 68 extending laterally from a sleeve 69 which is afiixed to a rock shaft 70 (Fig. 2) extending parallel to the lower shaft "44. This swinging of the carrier 67 imparts loop taking and shedding movements to the looper 60. Through rocking of the shaft 70, looper 60 is given its needle avoid movements. The shaft 70 is rocked in well known manner through action of an eccentrictnot shown) mounted on the shaft 44.

The work feeding devices may suitably be of the character disclosed in the patent to Peterson et al. No. 2,577,430, granted December 4, 1951. It may comprise a feed dog 71 (Fig. 1) carried by a feedbar72 (Fig. 2) pivotally mounted on a feed rocker '73. The latter may be rocked'by connections, inc'1uding .-an arm .74 and a pitman 75, from an adjustable eccentric 76 which may be adjusted to any desired position along a diameter of a disc 76 secured to the outer end of shaft 44. Another eccentric (not shown) on the shaft 44 is arranged to raise and lower the feed bar to carry the feed dog into and out .of engagement with --the work. A *presser foot 77 carried by a spring urged presser bar 78 of any suitable construction with the feed dog in advancing-the work.

As was pointed out hereinafter, torsional-vibrations may be set up in rotating or oscillating shafts of a sewing machine through inertial forces or through impactforces. In a machine of thetype illustrated, the most severe "torsionalvibrations are set up in the upper shaft 27 and-are caused primarily'by the inertia force arising from starting and stopping the needle bar 33 at "each end of ;its reciprocatory movement. These --vibrations may be suppressed-in accordance with the invention, by means of a dynamic torsional vibration damper arranged to operate in conjunction with the upper shaft 27 and tuned to suppress vibrations at the critical frequency of the shaft 27 and its driving connections. It should be'observed that the critical frequency of shaft 27 alone will gener ally fall well outside the operating speed -range of the machine so that there is little likelihood of serious-vibrations being setup at a frequency in the neighborhood of 'thisycritical frequency. However, when 'the shaft 2'7 is included in a power transmission system having appreciable flexibility, the critical frequency of the system may be lowered to a value at whichserious amplification of the torsional vibrations'imparted-to theshaft may'occur due to resonance effects resulting from the coincidence of the vibratory force frequency and the critical frequency. This flexibility may be introduced 'by means pfabelt coupling between the driven shaft and the driving shaft, as'in the machine illustrated in the drawings, or-rnay be introduced through other relatively flexible connections. -Similarly, flexibility 'may arise because of a belt or other flexible connection between the hand and-beltwheel and an electrical transmitter or otherprimary power source.

It has been found that, where it is desired to suppress torsional vibrations at a given frequency or withfin a narrow range of frequencies, the-combined detuning and damping action of an appropriate dynamic torsional vibration damper will produce satisfactory suppression of the undesired torsional vibrations. The term damping as used herein is synonymous with or includes absorbmgIn V V 4 Referring now particularly to Figs. 4 and 5, there is shown a dynamic torsional vibration damper associated withthe upper shaft 27. f-This damper comprises a "flexible -or resilient sleeve unit, designated generally as 80, fitted between anfinertia member -81 anda horizontally extending flange or hub portion 82 of .the sprocket 130. Theftex'ible sleeveunit '80 comprises a metal cylinder 83 surrounded bya flexible band 84, whichmayjhe made of rubber or like material. Band 84 is, in .turn,;sur rouuded byanot er me alicylind r .85- Th three-element i -l. 84 and 1.85 are formed .as an integral ,unit withsthe .cylinder 83 tightly mounted on the flange 82 for rotation hvithfihe .latter aud th cy 5 moun ed tight y within inertia member 81 for turning therewith. Cylinders 83 and 85 are bonded in any suitable way to the band 84. Slight relative turning of the two cylinders is permitted by the flexibility of the band 84.

The inertia member 81 is provided with a radially protruding flange portion 86 in which are provided surface grooves or slots 87 and 88 located, respectively, on opposite sides of the shaft 27. As best shown in Fig. 4, the slots 87 and 88 extend throughout the radial dimension of the inertia member 81 from the cylinder 85 to the outer edge of the flange portion 86. A pair of block weights or masses 89 and 90 are slidably mounted in the grooves 87 and 88, respectively, and are each provided with a threaded hole adapted to receive respective ends of a threaded adjusting rod 91. Blocks 89 and 90 are spaced equal distances from the center line of the shaft 27. The distance between the center line of the shaft 27 and the blocks 89 and 90 may be varied by turning the rod 91. To facilitate this turning, slots are provided in the ends of rod 91 and a serrated thumb wheel 92, which may be rigidly aifixed to or integral with the rod 91, is provided at the center thereof. Adjustment of radial spacing of blocks 89 and 90 may be achieved by inserting a screw driver or similar instrument in one of the end slots of the rod 91 or by turning the wheel 92. The arrangement is such that the blocks 89 and 90 are caused to move equal distances outwardly from the center line of the shaft 27 when the rod 91 is turned in one direction and inwardly toward the center line of the shaft 27 when the rod 91 is turned in the other direction. In order to prevent undesired turning of the rod 91 during operation of the sewing machine, the rod is engaged by a spring band 93 mounted in inertia member 81 by means of screws 94. The spring band 93, which is provided with a slot adapted to accommodate the serrated wheel 92,

presses against the rod 91 adjacent the center thereof so that turning of the rod 91 to adjust the radial spacing of blocks 89 and 90 must be effected against the friction created by the spring. Spring band 93 also retains the rod on the member 81 and holds the weights 89 and 90 within the respective slots 87 and 88. Movement of the blocks 89 and 90 toward or away from the axis of the shaft 27 changes the moment of inertia of member 81 and thus varies the vibrational frequency which will be effectively suppressed by the damper. The blocks 89 and 90 should be adjusted so that the damper is tuned to the critical frequency of the shaft 27 and its connected elements, including the belt 43.

When the periodic reversals of torsional force applied to the shaft 27, because of inertia or impact effects, or both, are at or close to the critical frequency of the entire driving connection including the shaft 27, the amplitude of the resulting vibrations will be great due to resonance effects. The closer the frequency of the periodic reversals of force is to the critical frequency, the larger will be the amplitude of the resulting, torsional vibrations. It should be observed that the critical frequency will be lowered to some extent by the presence of the inertia member 81 and the weights 89 and 90. The vibratory motions of the inertia elements of the damper, i. e., the inertia member 81, rod 91 and the blocks 89 and 90, in eflfect create an additional vibrational peak which is superimposed upon but out of phase with the normal peak created by the shaft and its driving connections. The effect is to reduce the magnitude of the normal peak and provide two spaced peaks of considerably smaller magnitude. By adjustment of the blocks the two peaks may be made of substantially equal magnitude. This reduces the maximum vibration to a minimum value by a detuning action. Damping or dissipation of vibratory energy, is produced by friction in the rubber or other resilient material forming the band 84, thereby further smoothing out the vibratory peaks.

Tunable damping means at this point in the sewing machine is desirable because the elasticity of the belt 43 6 changes with age. Tuning'or adjustment is also desirable because the elasticity of the band 84 tends to change with age and because it permits an initial adjustment to compensate for variations in elasticity between different belts and bands.

While the dynamic damper shown in Figs. 4 and 5 may be mounted on the shaft 27 partially within the belt sprocket 30, as shown, it may, if desired, be mounted on the shaft independently of the belt sprocket. In any event, the inertia member must be free to rotate or turn slightly in relation to the shaft and to the belt sprocket or other machine parts which are rigidly connected with the shaft. Such other machine part may be, for example, the combined hand and belt wheel 51 of Fig. l, the dy namic damper associated therewith operating in the same manner as the damper of Figs. 4 and 5.

Referring now to Fig. 6, in which the dynamic torsional vibration damper associated with the hand and belt wheel 51 of Fig. l is shown in greater detail, the damper comprises a flexible unit fitted between an inertia member 101 and an axially extending flange or hub portion 102 of the hand and belt wheel 51. The flexible unit 100, comprises a hollow metal cylinder 103 surrounded by a flexible annular band of rubber or other resilient material 104. Band 104 is surrounded by another metal cylinder 105. The three elements 103, 104 and 105 are formed as an integral unit with cylinder 103 mounted tightly on flange 102 for turning therewith, and cylinder 104 mounted tightly within inertia member 101 for turning with the latter. Inertia member 101 is formed with a radially extending flange 106 and a pair of surface grooves 107 and 108 are cut in the face of inertia member 101 and flange 106 on opposite sides of the shaft 44. A pair of block weights or masses 109 and 110 are slidably mounted in grooves 107 and 108, respectively, and are each provided with a threaded hole adapted to. receive a threaded adjusting rod 111. Blocks 109 and 110 are spaced equally from the axis of the shaft 44 and the radial spacing between this axis and the blocks 109 and 110 may be varied by turning the rod .111. In order to facilitate turning of the rod 111, a serrated thumb wheel 112 integral with the rod is provided at the center thereof. In order to prevent accidental turning of the rod 111 during operation of the sewing machine, the rod is engaged by a spring band 113 mounted on inertia member 101. The spring band also retains the rod on the inertia member and the blocks 109 and 110 in the respective slots 107 and 108. When assembled in a sewing machine, the damper of Fig. 6 may conveniently be adjusted by turning the thumb wheel 112 after first removing a cover plate 114 which is affixed to the hand and belt wheel 51 by means of screws 115. If desired, an opening 114:: may be provided in the cover plate to enable turning of the thumb wheel without removing the cover plate.

The damper of Figs. 4 and 5 may conveniently be adjusted by removing a cover member 116 (Fig. 1), at the top of the standard 24, and inserting a screw driver or similar instrument into the slot in one of the ends of the rod 91. The sewing machine with the cover plate 116 removed to render the upper shaft damper accessible is illustrated in Fig. 3.

.A dynamic type damper, constructed as shown in Figs. 4, 5,- and 6, has been found to suppress approximately 80% of the torsional vibrations at the critical frequency for which the damper is tuned. Dampers of this type may be incorporated in different parts of the sewing machine as needed and each is tuned to the critical frequency of the associated machine elements.

In the sewing machine of Fig. 1, the tuning of the damper associated with shaft 27 will be dependent primarily upon the mass and flexibility of the shaft 27 and the belt 43. The tuning of the damper associated with the shaft 44 will be dependent primarily upon the mass and flexibility of the shaft 44 and the belt or other means interconnecting the hand and belt :wheel filmwith the source of power. g

While theiinvention has been described iii-connectio V with aspecificillustrative example thereof and in a spebelt for driving said shaftrand connections from said shaft,

to said elements, at least .aportion ,of said driving .con nections including asaid belt and vsaid shaft and its associated connections forming an independently vibratahle system having a 'given critical frequency, means in said driving connections serving to apply periodic reversals of torsional force :to said shaft :in response to said reversals of movement of .atleast one of said elements and tending to create torsionalvibrations insaid shaft at substantially said critical frequency, and means for suppressing said torsional vibrations in said shaft comprising an inertia member arranged to rotate with said shaft and resiliently coupled thereto, said inertia member having relatively :adjustable parts adapted for adjustment to vary the natural frequency thereof to correspond substantially with said critical frequency.

2. In a 'high speed sewing machine having a plurality of cooperating stitch forming and work feeding elements, at least one of said elements being subjected -to reversals of movement in the :course of their operation, driving connections for said elements including a rotating shaft and a belt for driving said shaft, at least .a poitio-n of said driving connections forming an independently vibratable system having a given critical frequency and means in said driving connections serving to apply periodic -reversals of torsional force to said shaft in response to said reversals of movement of at least one of said elements and thereby tending to create torsional vibrations in said shaft at substantially said critical frequency, means for suppressing said torsional vibrations in said shaft comprising a resilient mass arranged to rotate with said shaft, -an inertia member arranged to rotate with said shaft and resiliently coupled thereto through ,said resilient ,mass, and manually adjustable means associated with said inertia member for varying the m ment of inertia thereof about the center of rotation of said inertia member so that its natural frequency may "be ,made' to correspond substantially with said criticalfrequency.

7 3.. In a high speed sewing machine having a plurality of cooperating stitch forming and work feeding elements, at least one of said elements be'in gsubjected to reversals of movement "in the course of their operation, driving connections for said elements including a rotating shaft and a belt for driving saidshafi, at least 'a portion of said driving connections "forming an independently vibratablc system having a given oritica'l frequency and means in said driving connections serving to apply periodic reversals of torsional force to said shaft in response to said reversals of movement :ofat least one of said elements and thereby tending ;to createtorsional vibrations insaid shaft at substantially said critical frequency, means for suppressing said torsional vibrations in said shaftcomprising-an inertia member arranged to rotate .with said shaft and resiliently coupled thereto, and means associated with said inertia ,member for simultaneously and coordinately varyingthellocation of diametrically opposed portions of the-mass thereof to a predetermined position in relation; to the center of rotation of said inertia memher so that its natural frequency may be made to conespond substantially =witl1said critical frequency.

.- 4. In :ahigh speed sewing machine having a'plurality ofnooperafing stitchform'ing and work feeding elements,

atleas on of said elements being i bise ed to reversals of mo emen in the cour o hei ope at on, dri ing connections for saidelements including a rotatingshaft and a belt for driving said shaft, at least a portionofsaid driving connections forming an independently vibratahle system having a given critical frequency and means in saiddriving connections serving to apply/periodic reversals of torsional force to said shaft in response to said reversals of movement of at least one of said elements and thereby tending to create torsional vibrations shaft at substantially said critical frequency, means for suppressing said torsional vibrations in said shaft com prising an inertia member arranged to rotate with said shaft and resiliently coupled thereto, said inertia'member having a plurality of radially shiftable masses mounted the n in iam rical y opp sed relatio 'l the axi 9f said shaft, and manually operable means associated with said inertia member for varying thedocation of said masses in relation to the center of rotation of said ;inerti a membe so that its na ura frequ ncy may e made to correspond substantially with said critical frequency' 15- ,In a high sp d wi g m h ne i g a plu al ty of cooperating stitch forming and work feeding elements, at least one of said elements being subjected to reve als of movement inthe course of their operation, driving non nections for saidelementsincluding ,a rotating shaft and a .belt for driving said shaft, at least a portion of ,said driving connections forming -an independently vibratable system having ,a given critical frequency-and means. :in'

and-thereby tending to create torsional wibrations insaid shaft at substantially said critical frequency, means :for suppressing said torsional vibrations in said shaft comprising a resilient mass arranged to rotate with said shaft, an inertia member carried by said resilient mass/and arranged to be rotated thereby .with 'said shaft, and means associated with said inertia member .for simultaneously varying :the location of diametrically opposed portions of the mass thereof to predetermined positions in relationto the center of rotation of said 'inertia mem her so that its-natural frequency may be made to correspond substantially with said critical frequency.

6. In a high speed sewing machine hav ng a plurality of cooperating stitch forming and :work feeding elements, at least one of said elements being subjected to -reversals of movement in the course'of their operation, drivingconnections for said elements includinga rotating shaft and a belt for driving said shaft, at least aportion of said driving connections forming an independently vibratable system having'a given critical frequency and means kin said driving connections serving to apply periodic reversals of torsional force to said shaft iii-response tosaid reversals of-rn0vement of at least oneof said eleineirtsand thereby tending to oreatetorsional vibrations in saidshaft at substantially "said critical frequency, means forsuppressing said torsional vibrations in said shaft comprisingaresilient and -yieldable band arranged to ,rotateivvith said shaft, a balanced inertia memberlmounted on'said band andarranged to be rotated thereby with said shaft, and simultaneously adjustable masses mounted on said inertia member for varying the moment of inertia thereof about its center of rotation so that its natural-frequency may'bernade to correspond substantially withsaid critical frequency without disturbing "the balance of'sa'id member.

7. In a high speed sewing -machine having a plurality of cooperating stitch forming and work feedingelements, at least one of said elements being subjected toreversals of movement in the course of their operation, driving connections for-said elements including a rotating shaft and a belt for driving said shaft, at least aportion oflsla d driving connections forming an independently 'yibratable system having -a given critical frequency and means in Y said driving connections serving to apply periodic reversals of torsional force to said shaft in response to said reversals of movement of at least one of said elements and thereby tending to create torsional vibrations in said shaft at substantially said critical frequency, tunable means for suppressing said torsional vibrations in said shaft comprising an inertia member arranged to rotate with said shaft and resiliently coupled thereto, said inertia member having uniformly distributed parts that are relatively shiftable to tune said member to a desired frequency, and means associated with said inertia member for simultaneously and uniformly shifting said parts relative to the axis of said shaft to tune said inertia member so that its natural frequency may be made to correspond substantially with said critical frequency.

10 References Cited in the file of this patent UNITED STATES PATENTS 131,782 Randall Oct. 1, 1872 394,435 Baumann Dec. 11, 1888 2,028,459 Kjaer Jan. 21, 1936 2,267,581 Zonis Dec. 23, 1941 2,411,373 Holowenko Nov. 19, 1946 2,450,701 Wahlberg et al. Oct. 5, 1948 2,512,735 Beiere June 17, 1950 FOREIGN PATENTS 346,094 Germany Dec. 24, 1921 OTHER REFERENCES Vibration Problems in Engineering (Timoshenko) published by D. Van Nostrand Co. (New York) 1937 (page 274 relied on). (Copy in Scientific Library.) 

